A spectrum concept of schizophrenia is described which hypothesizes that classical schizophrenia results at least in part from a genetic diathesis; that certain other, usually milder, psychopathological states, which do not satisfy the classical criteria for the diagnosis of schizophrenia, represent varying clinical expressions of the same diathesis; and that all of these states can, therefore, be said to constitute a genetically based spectrum of schizophrenic disorders. This concept is, in its broad outlines, not new, but it has had an increasing impact within psychiatry in recent years. Most importantly, it provides a conceptual framework for a variety of schizophrenia studies, particularly those dealing with biological markers, genetic vulnerability, clinical phenomenology, and modes of inheritance. A program of research which has attempted to test the concept, and to identify, describe, and classify the components of the genetic spectrum, is reviewed. Its methodology makes possible an approach to schizophrenia which simultaneously takes into account its two most vexing problems--nosology and etiology.